I did not find anything relevant.
Also, the first installation has been done with the 32 bits version ahead
of moving to the 64 version. So I believe that the best solution is to
unsinstall everything (clearing also user data if they are not removed) and
restart a proper installation with the new installer.

I have two OS : XP32 and W7-64 onto two separate discs with a third one
just for data. So, I don't believe it might be any confusion regarding
W7-64
looking somewhere else than "C" when it is operating.